Aluminum-based bearing alloy

ABSTRACT

The present invention has been made by the fact that the wettability with lubricant increases when tin grains are broken within a certain range. In an aluminum-based bearing alloy containing from 2 to 20 mass % of tin, the tin grains in a sliding surface have a size not less than 20 μm 2  but not more than 50 μm 2  expressed in region partitioned areas of the tin grains measured in accordance with a region partitioning method.

FIELD OF THE INVENTION

The present invention relates to an aluminum-based bearing alloysuitable for a sliding member.

BACKGROUND OF THE INVENTION

A sliding bearing lined with an aluminum-based bearing alloy withexcellent fatigue resistance has been well accepted as a bearing for anautomobile engine. An Al—Sn-based bearing alloy and an Al—Sn—Si-basedbearing alloy are widely used as aluminum-based bearing alloys. Thealuminum-based bearing alloy thus contains tin (Sn) which is a soft andhas a low melting point. The role of tin is summarized as follows: Theelement, tin, is interspersed in the bearing alloy, and melts when thetemperature of the sliding surface becomes high, which prevents thealuminum from adhering to a counterpart shaft. When tin melts, the heatof fusion lowers the temperature of the sliding surface and preventsseizure.

Such a sliding bearing lined with the aluminum-based bearing alloy(hereinafter simply referred to as a bearing) is manufactured throughthe following processes sequentially: casting, rolling, press bonding,heat treatment, and machine (see JP-A-2002-120047, Paragraph 0004, forexample). In the casting process, an aluminum-based bearing alloy ismelted and cast into a plate. The cast plate-shaped aluminum-basedbearing alloy is rolled in the rolling process, and bonded underpressure to a steel plate (back metal layer) with an thin aluminum oraluminum alloy plate (bonding layer) interposed in the pressure bondingprocess, and thus a bearing forming plate is produced. The bearingforming plate is then annealed to increase the bonding strength betweenthe aluminum-based bearing alloy and the steel plate, and finallymachined to form a semi-cylindrical or cylindrical bearing.

As recent automobile engines tend to be increasingly lighter, smaller,and more high-powered, a bearing is required to have excellent fatigueresistance under a high surface pressure. To meet the requirement ofimproving fatigue resistance following methods have been employed:adding copper, manganese, vanadium, and other elements to thealuminum-based bearing alloy to strengthen the matrix (see e.g.JP-A-6-136475); adding chromium and zirconium to the aluminum-basedbearing alloy followed by a solution treatment to strengthen the matrix(see e.g. JP-A-2000-17363); and reducing the amount of tin from thealuminum-based bearing alloy. Since reducing the amount of tin toenhance fatigue resistance is an ordinary technology, no appropriateknown publications have been found.

SUMMARY OF THE INVENTION

When a bearing deforms due to misalignment when the bearing is assembledto a housing or insufficient rigidity of the housing in association withreduction in weight and size of an engine, an uneven contact phenomenonlikely occurs, in which the counterpart shaft comes into uneven and hardcontact with the bearing, especially in an early stage of the operation.In this case, a conventional aluminum-based bearing alloy has relativelylow hardness, and uneven contact simply results in elastic deformationand unlikely ends up with oil film shortage. Therefore, uneven contactsimply results in weak metal contact, and the aluminum-based bearingalloy quickly conforms to the counterpart shaft.

In an aluminum-based bearing alloy having improved fatigue resistance inaccordance with any of the methods described above, however, thealuminum-based bearing alloy has increased not only strength but alsohardness, which makes it difficult to deform against uneven contact, andlikely resulting in oil film shortage and metal contact. The metalcontact causes the portions under uneven contact to be heated and adhereto the counterpart shaft, resulting in seizure. In particular, a bearinghaving a reduced amount of tin likely suffers from seizure.

The present invention has been made in view of the above circumstances.An object of the present invention is to provide an aluminum-basedbearing alloy capable of improving seizure resistance while maintaininggood fatigue resistance.

When the sliding surface, which is the surface of the bearing alloy,comes into uneven contact with the counterpart shaft, seizure does notoccur if oil film shortage does not occur. Oil film shortage does notoccur when the sliding surface has excellent wettability with lubricant.The present inventors have conducted intensive experiments to obtain analuminum-based bearing alloy with good wettability, and found that thewettability of the sliding surface is increased when tin grains in thematrix in the aluminum-based bearing alloy are made fine within acertain range to increase the interface area between the matrix and eachof the tin grains.

Wettability depends on the magnitude of the surface energy. When thesurface energy at the sliding surface is increased, the wettability withthe lubricant increases. There is a boundary between each of the tingrains and the matrix in an aluminum-based bearing alloy, and there ishence interface energy. Reducing the size of the tin grains increasesthe interface area between each of the tin grains and the matrix,whereby the interface energy can be increased. The increased interfaceenergy increases the surface energy at the sliding surface and hence thewettability with the lubricant. The present inventors have also foundthat when the tin grains are too fine, increases the hardness of thealuminum-based bearing alloy and hence degrades the conformability.

Based on the results of the experiments described above, the presentinvention provides an aluminum-based bearing alloy containing not lessthan 2 mass% but not more than 20 mass % of tin. In the aluminum-basedbearing alloy, size of tin grains in a sliding surface is not less than20 μm² but not more than 50 μm² when expressed in an average regionpartitioned areas of the tin grains measured in accordance with a regionpartitioning method.

The region partitioning method involves, as shown in FIG. 1, drawing aline between each pair of adjacent tin grains 2 in a matrix 1 (in thepresent invention, a line that separates Voronoi polygons obtained byconverting the tin grains in an observation area) to define regionswhich each single tin particle 2 occupies (regions surrounded by thedrawn lines), calculating statistically the region areas, andquantitatively judging the region partitioned areas. That is, providedthat the content of tin is fixed, the size of tin grains correlates withthe number of tin grains; larger tin grains result in a smaller numberof grains and larger region partitioned areas (the average region areaoccupied by a single particle). In contrast, smaller tin grains resultin a greater number of grains and smaller region partitioned areas. Thesize of the region partitioned areas can therefore quantitatively showthe size of the tin grains.

According to the present invention having the above features, theaverage region partitioned area of the tin grains is not less than 20μm² but not more than 50 μm². Preferably, the region partitioned areasof at least 70% of the tin grains in an observation area are not lessthan 20 μm² but not more than 50 μm². More preferably, the regionpartitioned areas of at least 95% of the tin grains are not less than 30μm² but not more than 40 μm². The tin grains are fine grains whoseaverage size is within a predetermined range and have large boundaryareas with the matrix, whereby large surface energy at the slidingsurface and good wettability with lubricant are achieved. Therefore,although an aluminum-based bearing alloy has improved fatigue resistanceand, in general, accordingly an increased matrix hardness, the goodwettability likely prevents oil film shortage even when a counterpartmember comes into uneven and hard contact with the sliding surface. Theseizure resistance can therefore be improved while the good fatigueresistance is maintained.

The content of tin ranging not less than 2 mass % but not more than 20mass % leads to excellent conformability, excellent adherence preventionand reduction in temperature at the sliding surface due to melting oftin at high temperatures, as well as high strength, and resistance tohigh surface pressures. Moreover, the amount of tin ranging not lessthan 2 mass % but not more than 20 mass % is an appropriate content thatallows the region partitioned areas of the tin grains to range not lessthan 20 μm² but not more than 50 μm², and can also set the boundaryareas between the tin grains and the matrix to appropriate values. Morepreferably, the region partitioned areas of the tin grains range notless than 30 μm² but not more than 40 μm².

As a method for reducing the region partitioned areas of the tin grainswithin a range of not less than 20 μm² but not more than 50 μm², theprocessing strain is accumulated during the processes from the castingin which the aluminum-based bearing alloy is cast or heat treatment fortempering performed immediately after the casting, until heat treatmentperformed immediately before the aluminum-based bearing alloy ismachined into a bearing. Heat treatment for annealing is performed forthe first time before the aluminum-based bearing alloy is machined intothe bearing. Post-casting rolling for producing a plate having apredetermined thickness, rolling for bonding the plate to a steel plateunder pressure, and other rolling processes elongate the tin grainsalong the rolling direction. When the elongated tin grains are heated toa temperature of not lower than the melting point of tin in the heattreatment performed before the aluminum-based bearing alloy is machinedinto the bearing, the tin grains tend to shrink into spheres due totheir surface tension and hence separate into fine grains. In thisprocess, the greater the amount of strain accumulated by the rolling andother processes is, the finer is the tin grains.

In the present invention, the matrix may contain an intermetalliccompound made of aluminum and at least two other metallic elements. Thenumber of the intermetallic compound grains whose size is smaller than 1μm may be not less than 300 in an observation area of 20 μm×15 μm.

After the aluminum-based bearing alloy is cast and then, subject to theheat treatment for tempering, the intermetallic compound described aboveprecipitates during the heat treatment and helps for fine dispersion ofthe tin grains. That is, the intermetallic compound described aboveprevents dislocation movement in the matrix in the rolling and othermachining processes. At the periphery of the intermetallic compoundhaving the capability described above, strain is likely accumulated, andstrain energy necessary to recrystallize the tin grains is accumulated.Therefore, controlling the amount of the intermetallic compound cancontribute to dispersion of the fine tin grains. Furthermore, since theintermetallic compound is in the form of fine grains whose size issmaller than 1 μm, the matrix is strengthened and the fatigue resistanceis improved. The effects of the intermetallic compound described aboveare well brought out when the number of intermetallic compound grains isnot less than 300 in an observation area of 20 μm×15 μm.

In the present invention, the elements other than aluminum, that formthe intermetallic compound, may be at least two elements selected fromthe group consisting of manganese (Mn), vanadium (V), chromium (Cr),cobalt (Co), iron (Fe), nickel (Ni), tungsten (W), titanium (Ti), andzirconium (Zr). In this case, a total amount of the at least twoelements preferably ranges not less than 0.01 mass % but not more than 3mass %.

Manganese, vanadium, chromium, cobalt, iron, nickel, and tungsten aretransition elements and not only form in combination with aluminum, anintermetallic compound capable of preventing the dislocation movementdescribed above but also disperse by themselves in the matrix. Since theabove transition elements hardly solid solute, they are not expected tostrengthen the matrix by solid solution. However, the elements dispersedin the matrix are capable of securely fixing the intermetallic compoundin the matrix by affinity between the above elements and theintermetallic compound containing the same. Therefore, the intermetalliccompound advantageously provides a strong dislocation movementprevention capability to accumulate large strain energy.

The aluminum-based bearing alloy of the present invention may furthercontain not less than 2 mass % but not more than 6 mass % of silicon.Silicon works for lapping on a counterpart shaft to enhance seizureresistance, and becomes fine grains and disperses in the matrix toincrease the strength and improve the fatigue resistance.

Furthermore, the aluminum-based bearing alloy of the present inventionmay further contain at least one element selected from the groupconsisting of copper (Cu) zinc (Zn), and magnesium (Mg) a total amountof which is not less than 0.1 mass % but not more than 7 mass %.

Copper, zinc, and magnesium are solid-soluble elements and solid solutein the matrix to strengthen the matrix.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a conceptual schematic view for explaining a regionpartitioning method.

DETAILED DESCRIPTION OF THE INVENTION

An example of the present invention will be specifically describedbelow.

The aluminum bearing alloys were cast into plates, by the belt casterdisclosed in JP-A-2002-120047, which alloys had compositions shown inTable 1 below. The aluminum bearing alloys were heat-treated fortempering (for example, for 5 hours at approximately 350° C.). The heattreatment causes fine intermetallic compound grains to crystallize inthe aluminum bearing alloys containing the transition elements. Each ofthe cast aluminum bearing alloys was made to have a thickness of 18 mm.

TABLE 1 Number of intermetallic compound smaller Tin region than 1 μmpartitioned Composition Elongation Number/ area No. Al Sn Si Cu Mn V Fe% (20 × 15) μm² μm² Invention 1 Balance 20 — 1 — — — 5100 0 50 examples2 Balance 8 2.5 1 0.3 0.2 0.2 4000 380 45 3 Balance 8 2.5 1 0.2 0.1 0.24000 270 50 4 Balance 8 2.5 1 0.3 0.2 0.2 4200 390 35 5 Balance 8 2.5 10.3 0.2 0.2 4400 390 20 6 Balance 8 2.5 1 0.2 0.1 0.2 4400 270 25 7Balance 8 2.5 1 0.2 0.2 0.2 3900 300 50 Conventional 11 Balance 20 — 1 —— — 3300 0 115 examples 12 Balance 8 2.5 1 0.3 0.2 0.2 3300 370 85 13Balance 8 2.5 1 0.2 0.1 0.2 3300 260 100 14 Balance 8 2.5 1 0.3 0.2 0.23600 390 65 15 Balance 8 2.5 1 0.3 0.2 0.2 4600 380 15

Each of the cast and tempered aluminum bearing alloy plates was rolledby a roller and then bonded under pressure to a thin aluminum oraluminum alloy plate to produce a multilayer aluminum alloy plate. Themultilayer aluminum alloy plate was then bonded under pressure to aback-metal steel plate. A bearing forming plate (bimetal) comprised ofthree layers of the steel plate (back metal layer), the bonding aluminumor aluminum alloy plate (intermediate layer), and the aluminum bearingalloy plate (bearing alloy layer) was thus produced. The bearing alloylayer was 0.45 mm in thickness. The produced bearing forming plate washeated to anneal at a temperature of not lower than therecrystallization temperature of the matrix. All the rolling processesdescribed above were cold rolling.

Table 1 also shows the degree of machining (primarily cold rolling) onthe aluminum bearing alloy plates from the heat treatment for temperingto the annealing for recrystallization after the bearing forming platewas produced. The degree of machining is expressed in elongation rate.The elongation rate is a value obtained by dividing the length aftermachining by the length before machining and expressed in percentage.The strain energy generated by the machining and accumulated in each ofthe aluminum bearing alloy plates is the energy necessary forrecrystallization.

For references, in JP-A-2002-120047, each aluminum bearing alloy is 15mm in thickness immediately after the casting, and the elongation ratewhen each aluminum bearing alloy is rolled into 0.45 mm in thickness isapproximately 3300%, which significantly differs from the elongationrates of the examples of the invention.

When the aluminum bearing alloy plate is subjected to post-casting heattreatment for tempering, an intermetallic compound is generated anddispersed in the matrix as fine grains smaller than 1 μm. The fineintermetallic compound grains serve to prevent dislocation movement inthe matrix so as to accumulate strain at the periphery of theintermetallic compound grains. More strain energy necessary forrecrystallization that facilitates fine tin grains is thus accumulated.

Through annealing for recrystallization, tin grains in the matrix aremade in fine grains. The annealed bearing alloy layer was observed withan electron microscope, and the photographed image was used to measurethe size and the number of the intermetallic compound grains as well asthe region partitioned areas of the tin grains in accordance with theregion partitioning method. Table 1 shows the results.

The produced bearing forming plates were machined into halved bearings.A seizure test was conducted on the halved hearings under the conditionsshown in Table 2, and the results are shown in Table 3.

TABLE 2 Seizure test conditions Rotating speed 7200 rpm Test loadIncremented by 10 MPa every 10 minutes Lubricant temperature 100° C.Amount of lubricant 80 ml/min Lubricant VG22 Shaft material S55CEvaluation method seizure is judged to occur when temperature ofbackside of bearing becomes greater than 200° C. or torque variationcauses shaft-driving-belt to slip

TABLE 3 Maximum surface pressure that did not cause seizure No. MPaInvention examples 1 80 2 85 3 85 4 90 5 85 6 85 7 85 Conventionalexamples 11 65 12 75 13 70 14 75 15 70

From the measurement results in Table 1, each of the tin regionpartitioned areas in examples 1 to 7 of the invention is not less than20 μm² but not more than 50 μm², which is within an adequate range.Comparing the invention example 1 with the invention examples 2 to 7, itis seen that it is advantageous to have fine intermetallic compoundgrains smaller than 1 μm in order to reduce the tin region partitionedareas.

On the other hand, comparing the invention example 1 with theconventional example 11, both of which do not include intermetalliccompound, the machining elongation rate in the conventional example 11is smaller than that in the invention example 1. The smaller elongaterate in the conventional example 11 is believed to be the cause of thelarger tin region partitioned area. It is therefore found that, when nointermetallic compound is present, the elongation rate needs to be large(5100%, for example) to obtain fine tin grains. In the conventionalexamples 12 to 14, although they include intermetallic compounds, thetin region partitioned areas are larger than 50 μm² since the elongationrates are small. In the conventional example 15 including anintermetallic compound, the tin region partitioned area is 15 μm², whichis too small, since the elongation rate is too large. Therefore, when anintermetallic compound is present, it is believed that a reasonableelongation rate ranges not less than 3900% but not more than 4400% whilethe reasonable range depends on the amount of the intermetalliccompound. In the production method described in JP-A-2002-120047, eachcast aluminum bearing alloy plate is 15 mm in thickness (see paragraph[0030]), and the elongation rate when the plate is rolled into 0.45 mmin thickness is approximately 3300%.

In view of the invention examples 2 to 7 including intermetalliccompounds, it is advantageous for making the tin grails fine to includea greater number of intermetallic compound grains smaller than 1 μm.Furthermore, comparing the invention examples 2 and 3 with the inventionexamples 5 and 6, a greater elongation rate serves to obtain the finetin grains. From the invention examples 2 and 3, it is furtheradvantageous in reducing the size of the tin grains to include notsmaller than 300 intermetallic compound grains smaller than 1 μm in anobservation area of 20 μm×15 μm, when the elongation rate is small (thedegree of machining is small).

The results of the seizure test are now analyzed. Since the seizure testis carried out under the conditions of insufficient lubricant where anamount of lubricant supply is reduced, better seizure resistance isobtained as the bearing surface (the surface of the aluminum bearingalloy) has better wettability. The seizure test therefore measures thewettability of the aluminum bearing alloy at the same time.

According to Table 3, the invention examples 1 to 6 having the tinpartitioned areas ranging not less than 20 μm² but not more than 50 μm²are excellent in seizure resistance. In contrast, it is seen that theconventional examples 11 to 15 are inferior to the invention examples 1to 6 in terms of seizure resistance since the maximum surface pressuresthat do not cause seizure are lower. A probable reason therefor is asfollows. In each of the invention examples 1 to 6, since the tin grainsare fine, the wettability of the aluminum bearing alloy with thelubricant increases Therefore, even when the aluminum bearing alloycomes into uneven contact with a counterpart shaft, the good wettabilityallows the lubricant to be brought into the portion where the unevencontact occurs unlikely resulting in oil film shortage but resulting inimprovement in seizure resistance. Therefore, according to the presentinvention, although the fatigue resistance of the aluminum bearing alloycan be improved by adding copper and silicon or carrying out a solutionprocess, the seizure resistance can be improved at the same time whilethe good fatigue resistance is maintained.

1. An aluminum-based bearing alloy containing not less than 2 mass % butnot more than 20 mass % of tin, wherein tin grains in a sliding surfacehave a size not less than 20 μm² but not more than 50 μm² expressed inan average region partitioned area of the tin grains measured by aregion partitioning method.
 2. The aluminum-based bearing alloyaccording to claim 1, further containing an intermetallic compoundcomposed of aluminum and at least two other elements, wherein the numberof intermetallic compound grains having a size smaller than 1 μm is notless than 300 in an observation area of 20 μm×15 μm.
 3. Thealuminum-based bearing alloy according to claim 2, wherein the at leasttwo other elements that form the intermetallic compound in combinationwith aluminum is selected from the group consisting of manganese,vanadium, chromium, cobalt, iron, nickel, tungsten, titanium, andzirconium, a total amount of the selected elements being not less than0.01 mass % but not more than 3 mass %.
 4. The aluminum-based bearingalloy according to claim 1, further containing not less than 2 mass %but not more than 6 mass % of silicon.
 5. The aluminum-based bearingalloy according to claim 1, further containing at least one elementselected from the group consisting of copper, zinc, and magnesium, atotal amount of the selected element being not less than 0.1 mass % butnot more than 7 mass %.